1. Field of the Invention
The present invention relates to a solid state image sensing device and a manufacturing process thereof, and more particularly, it relates to a solid state image sensing device whose photosensitivity is improved by provision of a transistor for amplifying signals between a photoelectric transducing diode and a MOS metal oxide semiconductor switching transistor and a process of manufacturing the same.
2. Description of the Prior Art
Conventional solid state (semiconductor) image sensors (hereinafter referred to as "SIS") have been restricted to a MOS type and a CCD type. FIG. 1 shows an equivalent circuit diagram of a fundamental cell (hereinafter referred to as "cell") forming a picture element of a commercially available MOS type SIS and a circuit diagram of a transistor connected thereto for reading signals.
Referring to FIG. 1, the cell comprises a photoelectric transducing diode (photodiode) PD and a MOS switching transistor TRs. The photoelectric charge generated by incidence of a light in the photodiode PD is stored in an interconnection capacity C.sub.V by turning-on of a signal reading MOS transistor TRs, whose voltage is generated as a video output. A transistor TRp as shown by the phantom line in FIG. 1 functions to extract an overcurrent caused by an oversaturated light.
FIG. 2 is a time chart of a video output current. Since the interconnection capacity C.sub.V is smaller than 1/100 of the capacity C.sub.H, the signal current is made to be a fine current superposed on a clock noise as shown in FIG. 2, whereby the dynamic range of the video output is significantly restricted. Thus, it is necessary to enlarge the area of the photodiode PD, for providing sufficient photoelectric conversion current.
FIG. 3 is a cross-sectional view showing a cell of the conventional MOS type SIS as shown in the equivalent circuit of FIG. 1 and partially showing cells adjacent to both sides thereof.
Structure of the cell of the conventional MOS type SIS as shown in FIG. 3 is now described. In FIG. 3, a p-type well 2, functioning as an anode of the photodiode PD, is formed on an n.sup.- -type substrate 1, and selectively formed on the surface of the p-type well 2 is an n.sup.+ layer 3 which is a cathode of the photodiode PD while functioning as a source of the MOS switching transistor TRs. Further, the p-type well 2 is provided therein with an n.sup.+ layer 4 which functions as a drain of the MOS switching transistor TRs, such that a channel forming region is defined between the same and the n.sup.+ layer 3. An oxide film 5 is deposited on the p-type well 2, including the n.sup.+ layers 3 and 4, and a gate electrode 6, formed by polycrystal silicon, is provided on a gate oxide film, which is a part of the oxide film 5 located on the channel forming region of the MOS switching transistor TRs. Then, a layer insulating film 7 is formed over the oxide film 5 including the gate electrode 6, and a drain electrode 8 is provided in an opening formed through the oxide film 5 and the layer insulating film 7, to be in contact with the n.sup.+ layer 4. The gate electrode 6 is connected to an interlace circuit (not shown) while the drain electrode 8 is connected to the signal reading MOS transistor TRo (not shown in FIG. 3). The n.sup.+ layer 3, the p-type well 2 and the n.sup.- -type substrate 1 of the photodiode PD form the transistor TRp as shown by the phantom line in FIG. 1, which functions to extract an overcurrent caused by an oversaturated light.
Operation of the cell of the conventional MOS type SIS as shown in FIG. 3 is now described. When the photodiode PD formed by the p-type well 2 and the n.sup.+ layer 3 receives a light, a photoelectric charge is generated to be stored in the junction capacitance. Then the gate electrode 6 connected to the interlace circuit is turned on so that a current flows from the n.sup.+ layer 3, which is the source of the MOS switching transistor TRs, to the n.sup.+ layer 4 which is the drain of the MOS switching transistor TRs, to be fed through the drain electrode 8 to the signal reading MOS transistor TRo (not shown), which in turn is turned on to provide a video signal corresponding to the optical signal amount.
In the meantime, it is necessary to enlarge the area of the n.sup.+ layer 3 which is the cathode of the photodiode PD, for increasing the photoelectric current, namely, for increasing photosensitivity as hereinabove described.
However, since the incident area of the light to the SIS is determined by the optical system such as a lens, the area of the cell with a predetermined number of picture elements is inevitably limited, whereby the cathode area cannot be freely enlarged. Although it can be considered to amplify the output from the drain of the MOS switching transistor TRs for improving the photosensitivity, clock noises and fixed pattern noises are also amplified, leading to ineffective increase of the photosensitivity.